As illustrated in FIG. 1, a hollow charge constitutes, in a known manner, the destructive part of a projectile intended for the perforation of armour elements. This hollow charge comprises several elements.
Chiefly these are a priming element 13, a shield 14 and an explosive element 12. The whole assembly is packaged in a casing 11 closed off at one of its ends by the part 15, called the liner, intended to penetrate the armour attacked.
The liner 15, generally of substantially conical shape, is made of metal such as copper for example.
Upon the firing of the charge and under the action of the explosive, the liner 15 acquires a very significant kinetic energy and is imbued with a very significant and very fast expansion. Under the effect of this very significant deformation it takes the shape illustrated by FIG. 2. The liner then comprises two parts, a rear part 21 called the core and a front part 22 called the jet.
The jet represents the perforating part of the charge. Its dimensions and in particular the length are expanding according to a velocity gradient, the tip of the jet 23 being propelled at a velocity of around 8000 m/s, the tail of the jet 24 having a velocity of around 3000 m/s. The core 21 is for its part propelled at a velocity of around 1000 m/s.
As illustrated in FIG. 1, the jet 22 takes the form of a long rod of melting metal, with a diameter of the order of 2 mm, the surface of which exhibits, as shown by the enlargement, a slightly annulated appearence with bulges 26 and constrictions 25.
In the course of the motion the core tends to take a flattened shape whose dimensions, significant with respect to the diameter of the jet are such that it hardly participates in the perforation.
On account of the kinetic energy that it possesses, the jet is capable of penetrating metal armour several decimetres thick. Effective inert protection against projectiles equipped with a hollow charge therefore consists in increasing the thickness of the armour and therefore turns out to be very penalizing in terms of weight.
To protect mobile structures, such as armoured vehicles, against such projectiles, while avoiding an excessive weight increase, it known to use electrical devices whose aim is to destructure the jet as rapidly as possible during its penetration by disintegrating it.
For this purpose, a known principle consists in implementing a structural destabilization of the jet (breakup), associated with melting/vaporization under the effect of the passage of an electric current, the melting of the jet being the predominant effect in the neutralization of the hollow charge. This known principle is illustrated by FIG. 3.
According to this known principle, the protective structure implemented consists essentially of two metal plates placed on the surface to be protected and constituting two electrodes connected to a battery of charged capacitors which apply a very high voltage between the two plates. When the hollow charge jet develops, it short-circuits the two electrodes and a very intense current is established progressively in the metal jet. The effect of this current is to heat the jet up to vaporization.
To ensure total effectiveness, it is necessary for the destruction of the jet to take place during its passage between the two electrodes. The elements of the jet having crossed the space between the two plates without damage are no longer subjected to the electric current and are therefore no longer liable to be destroyed. This is true in particular for the beginning of the jet. They may therefore cause damage, the protection then being partially ineffective.
Given that the jet penetrates the structure at very high velocity, it is readily appreciated that the success of the destruction of the jet depends on the energy imparted by the Joule effect and on the time taken to impart this energy to the jet. This energy is in practice proportional to the square of the intensity of the current and to the duration of passage between the two electrodes.
Thus, at the beginning of penetration, the maximum current is not established on account of the stray inductive elements present in the circuit constituted by the plates and the jet. The intensity of the current which then flows around the jet is weak so that the head of the jet is not destroyed. The known principle of the prior art does not therefore ensure complete destruction of the charge and leaves intact the part with the most energy which may carry out its perforation function without encountering a complete obstacle.
To alleviate this problem a certain number of solutions have been envisaged. The objective being to allow a current to flow around the jet head after the latter has left the space between the plates.
A first solution envisaged consists in installing a massive metal structure behind the earth electrode and linked electrically to the latter. The drawback of such a solution is the weight of the structure thus formed, hardly compatible for example with a mobile structure of armoured vehicle type.
Another solution illustrated by FIG. 4 consists in the placing in abutment with the internal plate of a structure made of metal plates disposed in parallel planes perpendicular to the plane of the plate. This architecture, much lighter than that described previously, allows a current to be made to flow around the head of the jet after the latter has left the space between the two plates. On the other hand, its effectiveness depends on the orientation of the jet with respect to the plane defined by the plates and the spacing of the plates with respect to one another. Likewise the intensity of the current established between the jet head and one or other plate is dependent on this separation and on the diameter of the jet and the position of the jet with respect to one or other plate. As far as this architecture is concerned the illustration of FIG. 4 corresponds however to a favourable case where the jet progresses between two plates, the distance between these plates being suitably adjusted with respect to the diameter of the jet. In order to get close to this favourable case as often as possible, it is necessary for the structure to comprise a large number of plates close together, thus reviving the problem of the weight of the whole assembly.